162 MISCELLANEOUS OBJECTIONS TO THE [CHAP. VII. 



ference and centre sometimes differ greatly in form, colour, and 

 other characters. In Carthamus and some other Composite the 

 central achenes alone are furnished with a pappus ; and in Hyoseris 

 the same head yields achenes of three different forms. In certain 

 Umbelliferee the exterior seeds, according to Tausch, are ortho- 

 spermous, and the central one ccelospermous, and this is a character 

 which was considered by De Candolle to be in other species of the 

 highest systematic importance. Prof. Braun mentions a Fumari- 

 aceous genus, in which the flowers in the lower part of the spike 

 bear oval, ribbed, one-seeded nutlets; and in the upper part of 

 the spike, lanceolate, two-valved, and two-seeded siliques. In 

 these several cases, with the exception of that of the well developed 

 ray-florets, which are of service in making the flowers conspicuous 

 to insects, natural selection cannot, as far as we can judge, have 

 come into play, or only in a quite subordinate manner. All these 

 modifications follow from the relative position and inter-action of 

 the parts ; and it can hardly be doubted that if all the flowers and 

 leaves on the same plant had been subjected to the same external 

 and internal condition, as are the flowers and leaves in certain 

 positions, all would have been modified in the same manner. 



In numerous other cases we find modifications of structure, 

 which are considered by botanists to be generally of a highly 

 important nature, affecting only some of the flowers on the same 

 plant, or occurring on distinct plants, which grow close together 

 under the same conditions. As these variations seem of no special 

 use to the plants, they cannot have been influenced by natural 

 selection. Of their cause we are quite ignorant ; we cannot even 

 attribute them, as in the last class of cases, to any proximate 

 agency, such as relative position. I will give only a few instances. 

 It is so common to observe on the same plant, flowers indifferently 

 tetramerous, pentamerous, &c., that I need not give examples; 

 but as numerical variations are comparatively rare when the parts 

 are few, I may mention that, according to De Candolle, the flowers 

 of Papaver bracteatum offer either two sepals with four petals 

 (which is the common type with poppies), or three sepals with six 

 petals. The manner in which the petals are folded in the bud is 

 in most groups a very constant morphological character; but 

 Professor Asa Gray states that with some species of Mimulus, 

 the aestivation is almost as frequently that of the Rhinanthidese 

 as of the Antirrhinidese, to which latter tribe the genus belongs. 

 Aug. St. Hilaire gives the following cases : the genus Zanthoxylon 

 belongs to a division of the Piutaceae with a single ovary, but in 

 some species flowers may be found on the same plant, and even 

 in the same panicle, with either one or two ovaries. In Helian- 

 themum the capsule has been described as unilocular or 3-locular ; 

 and in H. mutabile, "Une lame, 2^ us ou moins large, s'etend 



